This invention relates to a beamforming system for a linear phased array antenna which can be used in a monopulse radar or communications system. Monopulse radars are tracking radars which can derive target angle tracking error on the basis of a single pulse by measuring the relative phases or amplitudes of target echoes received in two closely spaced beams wherein both beams result from a single transmitter pulse. This technique eliminates tracking errors caused by pulse-to-pulse amplitude fluctuations caused for example by target motion, in systems which require several pulses to obtain target error information. Monopulse radars often utilize phased array antennas which can be electronically steered for scanning purposes and can be easily adapted to measure the phase or amplitude differences required to derive the angular tracking error.
Phase array antennas require that the power applied to the elements thereof be tapered off toward the ends thereof if excessive sidelobes in the antenna pattern are to be avoided. Further, the use of digital phase shifters for beam steering or other purposes introduces quantization errors which degrades the antenna performance. The present invention provides a broadband low loss power distribution network for achieving a desired amplitude and phase distribution to the elements of a phased array antenna in a way which provides low sidelobes over an octave band and also minimizes the adverse effects of the use of the digital phase shifters by decorrelating the aforementioned phase shifter quantization.
There are two fundamentally different ways to form the patterns of a phase array. One is by means of a conventional corporate feed with a variable phase shifter between each output network and the radiating elements of the array. The other is through the use of a dual space-fed lens system like that of the present invention substituted for the corporate feed. The concept of the invention is clearly superior in most respects, including relative cost effectiveness, relative efficiency, relative weight efficiency and low sidelobe levels over an octave band.
Isolated unequal power divisions are required when using a corporate feed to generate a suitable amplitude taper for realization of low sidelobes. Corporate feeds require power dividers which are isolating four port junctions, and wideband isolated unequal power dividers using waveguide components are almost impossible to implement. Thus the ability of conventional corporate feeds to maintain low sidelobes for very wide bandwidth arrays is limited. Furthermore, this conventional apparatus is neither cost effective nor weight efficient.